arXiv subjects

Soumya Chakrabarti

Publications and source records attributed to Soumya Chakrabarti.

At least 37 records · Page 2Linked to original sources

arXiv2024

Cosmic Acceleration and the notion of Alternative Vacuum in Nash Theory

We argue that Nash theory, a quadratic theory of Gravity, can describe a late-time cosmic acceleration without any exotic matter or cosmological constant. The observational viability of an exact cosmological solution of Nash theory is adjudged using a Markov chain Monte Carlo simulation and JLA+OHD+BAO data sets. Departures from standard ΛCDM cosmology are noted and analyzed. We prove that the Nash vacuum dynamics is equivalent to the dynamics of an Einstein vacuum plus a self-interacting Higgs scalar field, only if a mild evolution of the Higgs Vacuum Expectation Value is allowed. This leads to variations in the mass scales of fundamental particles and the fine structure constant. The variations are found to fit nicely with the analysis of molecular absorption spectra from a series of Quasars.

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arXiv2024

Self-Similar Collapse in Painleve-Gullstrand Coordinates

We report a family of self-similar exact solutions in General Relativity. The solutions are found in a Painleve-Gullstrand coordinate system but can also be transformed smoothly into a diagonal form. The solutions represent a gravitational collapse leading to three possible outcomes, depending on the parameter space : (i) a collapse followed by a bounce and dispersal of the clustered matter distribution, (ii) a rapid collapse followed by a bounce and an eventual re-collapse, and (iii) a standard collapse leading to zero proper volume. Profiles of the energy conditions are studied for all of the scenarios, and it is noted that a bounce is usually associated with a violation of the Null Energy Condition. It is found that more than one null surfaces (apparent horizons) can develop during the collapse. We also discuss that for a general metric tensor having a conformal symmetry, some regions of the parameter space allows a formation of null throat, much like a wormhole. Matching the metric with a Schwarzschild metric in Painleve-Gullstrand form leads to the geodesic equation for a zero energy falling particle in the exterior.

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arXiv2024

Phase Transition and Thermodynamic Stability in an Entropy-driven Universe

Motivated by the notion that the mathematics of gravity can be reproduced from a statistical requirement of maximal entropy, we study the consequence of introducing an entropic source term in the Einstein-Hilbert action. For a spatially homogeneous cosmological system driven by this entropic source and enveloped by a time evolving apparent horizon, we formulate a modified version of the second law of thermodynamics. An explicit differential equation governing the internal entropy profile is found. Using a Hessian matrix analysis of the internal entropy we check the thermodynamic stability for a ΛCDM cosmology, a unified cosmic expanson and a non-singular ekpyrotic bounce. We find the mathematical condition for a second order phase transition during these evolutions from the divergence of specific heat at constant volume. The condition is purely kinematic and quadratic in nature, relating the deceleration parameter and the jerk parameter that chalks out an interesting curve on the parameter space. This condition is valid even without the entropic source term and may be a general property of any phase transition.

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arXiv2023

The fate of a Quantum-Corrected Collapsing Star in General Relativity

We incorporate some corrections inspired by loop quantum gravity into the concept of gravitational collapse and propose a complete model of the dynamic process. The model carries the essence of a mass-independent upper bound on the curvature scalars originally found as a crucial feature of black holes in loop quantum gravity. The quantum-inspired interior is immersed in a geometry filled with null radiation and they are matched at a distinct boundary hypersurface. The ultimate fate of the process depends on inhomogeneities of the metric tensor cofficients. We find a critical parameter $λ$ embedded in the inhomogeneity of the conformal factor of the interior metric. Examples with $λ< 0$ enforce an eventual collapse to singularity and $λ> 0$ cases produce a non-singular collapse resulting in a loop-quantum-corrected Schwarzschild geometry modulo a conformal factor. Interestingly, for $λ< 0$ as well, there exist situations where the quantum effects are able to cause a bounce but fall short of preventing the ultimate formation of singularity. The trapped surface formation condition is studied for $λ<0$ case to infer about the visibility of the final singularity. Interestingly, we find a possibility of formation of three horizons during the course of the collapse. Eventually all of them merge into one single horizon which envelopes the final singularity. For the non-singular case, there is a possibility that the sphere can evolve into a wormhole throat whose radius is found to be inversely proportional to the critical parameter $λ$. Depending on the nature of evolution and the shell regions, the collapsing shells violate some standard energy conditions which can be associated with the quantum inspired corrections.

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arXiv2023

Can a variation of Fine Structure Constant influence the fate of Gravitational Collapse?

We show that it is possible to steer clear of a spacetime singularity during gravitational collapse by considering the time-variation of a fundamental coupling, in this case, the fine structure constant α. We study a spherical distribution of cold dark matter coexisting with other fluid elements, collapsing under its own gravity. Dark matter is written as a scalar field interacting with electrically charged matter. This leads to a time variation of α and as a consequence, a breakdown of local charge conservation within the sphere. The exterior has no such field and therefore, Einstein's GR and standard equivalence principles remain valid. We derive the lowest possible bound on the collapse of this sphere beyond which there is a bounce and dispersal of most of the accumulated matter. We discuss the critical behavior of the system around this point and show that the bound is connected to a length scale of the order of Planck, introduced in theory for dimensional requirements.

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arXiv2023

The River Model of Gravitational Collapse

We show that the transformation of a time-evolving spherically symmetric metric tensor into a Painleve-Gullstrand-Lemaitre form brings forth a few curious consequences. The time evolution describes a non-singular gravitational collapse, leading to a bounce and dispersal of all the clustered matter, or a wormhole geometry for certain initial conditions. The null convergence condition is violated only at the onset of bounce or the wormhole formation. As an example, the requirements to develop a Simpson-Visser wormhole/regular black-hole geometry is discussed. The solution can be regarded as a new time-evolving twin of sonic dumb holes found in analog gravity.

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arXiv2022

Screening Mechanism and Late-time Cosmology: Role of a Chameleon-Brans-Dicke Scalar Field

We discuss a way in which the geometric scalar field in a Brans-Dicke theory can evade local astronomical tests and act as a driver of the late-time cosmic acceleration. This requires a self-interaction of the Brans-Dicke scalar as well as an interaction with ordinary matter. The scalar field in this construct acquires a density-dependent effective mass much like a Chameleon field. We discuss the viability of this setup in the context of the Equivalence Principle, Fifth Force, and Solar System tests. The cosmological consistency is adjudged in comparison with observational data from recalibrated light curves of type Ia supernova (JLA), the Hubble parameter measurements (OHD), and the Baryon Acoustic Oscillation (BAO). We deduct that the astrophysical constraints indeed favor the existence of a mild scalar-matter interaction in the Jordan Frame.

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arXiv2022

Scalar-Fermion Interaction as the Driver of Cosmic Acceleration

We argue that an interacting scalar-fermion distribution can be used to demonstrate the cosmic acceleration in General Relativity. The interaction is of Yukawa nature and it drives the fermion density to decay with cosmic time. The consistency of the model is established through, (a) a generalization of the Om(z) parameter, the present matter density contrast of the universe, and (b) a comparison of theoretical results with observational data, using a Markov Chain Monte Carlo code. A simple model of unified cosmic expansion history is also discussed. There are more than one instances where the universe goes through a transition between subsequent epochs with different patterns of cosmic expansion. These patterns impose some constraints on the scalar-fermion interaction profile and on the overall cosmological dynamics.

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arXiv2022

On Generalized Theories of Varying Fine Structure Constant

We work with a class of scalar extended theory of gravity that can drive the present cosmic acceleration as well as accommodate a mild cosmic variation of the fine structure constant $α$. The motivation comes from a vintage theory developed by Bekenstein, Sandvik, Barrow and Magueijo. The $α$ variation is introduced by a real scalar field interacting with charged matter. We execute a cosmological reconstruction based on a parametrization of the present matter density of the Universe. Observational consistency is ensured by comparing the theoretical estimates with JLA + OHD + BAO data sets, using a Markov chain Monte Carlo simulation. An analysis of molecular absorption lines from HIRES and UVES spectrographs is considered as a reference for the variation of $α$ at different redshifts. Two examples are discussed. The first explores a field-dependent kinetic coupling of the scalar field interacting with charged matter. The second example is a generalized Brans-Dicke formalism where the varying $α$ is fitted in as an effective matter field. This generates a simultaneous variation of the Newtonian constant $G$ and $α$. The pattern of this variation can have a crucial role in cosmic expansion history.

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arXiv2021

Cosmic Variation of Proton to Electron Mass Ratio with an interacting Higgs Scalar Field

We discuss that it is quite possible to realize the smooth transition of the universe between a matter/radiation dominated deceleration and a dark energy dominated acceleration, even with a variation of proton-to-electron mass ratio. The variation is incorporated into the theory of gravity using a cosmological Higgs scalar field with a non-trivial self-interaction potential, leading to a varying Higgs vacuum expectation value (VEV). This matches well with the data from molecular absorption spectra of a series of Quasars. In comparison with late-time cosmology, an observational consistency is reached using a Markov chain Monte Carlo simulation and JLA+OHD+BAO data sets. We find that the pattern of variation is embedded within the evolving Equation of State (EOS) of the scalar Dark Energy/Matter components, but leaves negligible trace on the effective EOS of the system. We discuss three cases of scalar extended theory of gravity, (a) a minimally coupled scalar, (b) a non-minimally coupled scalar and (c) a generalized Brans-Dicke setup. We also give a toy model of a unified cosmic history from inflation to the present era and discuss how the Higg VEV might have changed as a function of look back time.

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arXiv2021

A wormhole geometry from gravitational collapse

We discuss a proposal on how gravitational collapse of a NEC (Null Energy Condition) violating spherically symmetric fluid distribution can avoid the formation of a zero proper volume singularity and eventually lead to a Lorentzian wormhole geometry. Our idea is illustrated using a time-evolving wormhole spacetime in which, we show how a collapsing sphere may never reach a zero proper volume end-state. The nature of geodesic congruences in such spacetimes is considered and analyzed. Our construction is inspired from a recently proposed static wormhole geometry, the multi-parameter Simpson-Visser line element, which is known to unite wormholes and black holes (regular and singular) in a single framework.

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arXiv2020

Cosmic Acceleration in an Extended Brans-Dicke-Higgs Theory

We consider an extended scalar-tensor theory of gravity where the action has two interacting scalar fields, a Brans-Dicke field which makes the effective Newtonian constant a function of coordinates and a Higgs field which has derivative and non-derivative interaction with the lagrangian. There is a non-trivial interaction between the two scalar fields which dictates the dominance of different scalar fields in different era. We investigate if this setup can describe a late-time cosmic acceleration preceded by a smooth transition from deceleration in recent past. From a cosmological reconstruction technique we find the scalar profiles as a function of redshift. We find the constraints on the model parameters from a Markov Chain Monte Carlo analysis using observational data. Evolution of an effective equation of state, matter density contrast and thermodynamic equilibrium of the universe are studied and their significance in comparison with a LCDM cosmology is discussed.

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arXiv2020

Geodesic Congruences and a Collapsing Stellar Distribution in f (T ) Theories

Teleparallel Gravity (TG) describes gravitation as a torsional- rather than curvature-based effect. As in curvature-based constructions of gravity, several different formulations can be proposed, one of which is the Teleparallel equivalent of General Relativity (TEGR) which is dynamically equivalent to GR. In this work, we explore the evolution of a spatially homogeneous collapsing stellar body in the context of two important modifications to TEGR, namely f (T) gravity which is the TG analogue of f (R) gravity, and a nonminimal coupling with a scalar field which has become popular in TG for its effects in cosmology. We explore the role of geodesic deviation to study the congruence of nearby particles in lieu of the Raychaudhuri equation. We find f (T) models that satisfy the null energy condition and describe interesting collapse profiles. In the case of a nonminimally coupled scalar field, we also find potential collapse models with intriguing scalar field evolution profiles.

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arXiv2020

A Collapsing Distribution of Axion Dark Matter

The manuscript deals with an interacting scalar field that mimics the evolution of the so-called Axion Scalar Dark Matter or Axion like particles with ultra-light masses. It is discussed that such a scalar alongwith an ordinary fluid description can collapse under strong gravity. The end state of the collapse depends on how the Axion interacts with geometry and ordinary matter. For a self-interacting Axion and an Axion interacting with geometry the collapse may lead to a zero proper volume singularity or a bounce and total dispersal of the Axion. However, for an Axion interacting with the ordinary fluid description, there is no formation of singularity and the Axion field exhibits periodic behavior before radiating away to zero value. Usually this collapse and dispersal is accompanied by a violation of the Null Energy Condition.

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arXiv2020

Reconstruction of $f(R)$ lagrangian from a massive scalar field

We present a reconstruction of the Lagrangian for $f(R)$ gravity by using a massive scalar field. The scalar field is minimally coupled to the action of $f(R)$ gravity. We demonstrate the use of a theorem based on invertible point transformation of anharmonic oscillator equation that has only recently been applied to gravitational physics. This avenue gives us a direct way to solve the field equations for a few well-known self-interaction potential of the scalar field. A few cases which do not fall in the regime of the theorem are also discussed. The $f(R)$ models are discussed in the context of recent cosmological observations and viability issues.

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arXiv2019

Self Similar Collapse and the Raychaudhuri equation

The role of the Raychaudhuri equation in studying gravitational collapse is discussed. A self-similar distribution of a scalar field along with an imperfect fluid in a conformally flat spacetime is considered for the purpose. The general focusing condition is found out and verified against the available exact solutions. The connection between the Raychaudhuri equation and the critical phenomena is also explored.

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arXiv2019

Exact Solutions and Accelerating Universe in Modified Brans-Dicke Theories

Exact solutions are studied in the context of modified Brans-Dicke theory. The non-linearity of the modified Brans-Dicke field equations is treated with the Euler-Duarte-Moreira method of integrability of anharmonic oscillator equation. While some solutions show a forever accelerating nature, in some cases there is a signature flip in the evolution of deceleration parameter in recent past. Importance of these latter models are studied in the context of late-time acceleration of the universe. Constraints on the model parameters are obtained from Markov Chain Monte Carlo (MCMC) analysis using the Supernova distance modulus data, observational measurements of Hubble parameter, Baryon acoustic oscillation data, and the CMB Shift parameter data.

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arXiv2019

On Reconstruction of Extended Teleparallel Gravity from the Cosmological Jerk Parameter

Teleparallel gravity offers a path to resolve a number of longstanding issues in general relativity by re-interpreting gravitation as an artifact of torsion rather than curvature. The present work deals with cosmological solutions in an extension of teleparallel gravity. A reconstruction scheme of the theory has been proposed based on the cosmological jerk parameter. The work contains analysis of ensuing cosmological parameters for different viable models and the stability of the models against cosmic time through an investigation of perturbation of matter overdensity and the hubble parameter.

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